The present invention relates to a method of providing a conductor pipe to an opening portion of a well such as an oil well, gas well, geothermal power well or the like.
Conventionally, such wells have been bored by means of the rotary drilling machine (hereinafter referred to as drilling rig) to a depth of several kilometers.
FIGS. 1(a) to (f) illustrates initial stages of a typical example of the conventional method for drilling a geothermal well by means of the drilling rig, in which an opening portion of the well is formed. In this drilling method, firstly the ground A is, as shown in FIG. 1(a), drilled by rotating a drilling pipe 1 and a bit 2 attached at the lower end of the drilling pipe 1 while a mud is forcedly circulated through the drilling pipe 1 and the bit 2 to thereby transport the cutting to the outside of a well B. As the drilling proceeds further, the drilling pipe 1 is coaxially connected to subsequent pipes. The well B is thus formed to a predetermined depth as shown in FIG. 1(b). Then, a conductor pipe 3 is inserted into the well B (FIG. 1(c)) after the drilling pipe 1 and the bit 2 are withdrawn from the well B. Thereafter, an injection pipe 5 is inserted into the conductor pipe 3 placed in position and a solidifiable filler such as a cementinuous slurry C is then pumped from a filler pumping unit 4 through the injection pipe 6 to fill the annular space between the conductor pipe 3 and the wall of the well B as shown in FIG. 1(d). Finally, the conductor pipe 3 is, as illustrated in FIG. 1(e), fixed in the opening portion of the well B by the setting of the filler C to thereby complete the forming of the opening portion of the well. The well B is drilled to a predetermined depth by repeating the above-described method with the bits and the conductor pipes of smaller diameters.
This prior art drilling method has the following problems in forming the opening portion of the well B. The depth of the opening portion of the well is generally 30 to 50 meters and hence this prior art method can be poor in drilling efficiency since the drilling pipe 1 and the bit 2 may not provide by their weight a bit load sufficient for drilling the well. The opening portion of the well B is formed in the surface portion of the ground, the surface portion containing various layers such as a silt layer, cobblestone layer, bounding stone layer and rock bed. Particularly, the bit 2 is, according to the prior art, repelled by the cobblestone layer and the bounding stone layer during the drilling, so that the drilling perpendicularity of the well B is degraded and the diameter thereof becomes larger than the design diameter. These cause the consumption of the filler C larger than the design amount thereof. For example, for about 40 m deep opening portion of a typical geothermal power well constructed according to this prior art method, the maximum drilling perpendicularity is about 1/100 and the filler loss ratio is about 20%. Further, this method has disadvantages in that the mud, circulating in the well B during drilling, escapes into clearances, communicating to the well, between adjacent layers to thereby weaken the supporting ground of the drilling rig and wash out the wall of the well.